In recent years, flat panel displays (FPDs) have become mainstream as image display devices, the FPDs being typified by a liquid crystal display, a plasma display, an OLED display, and the like. Further, progress is being made toward reduction in weight of those FPDs. Therefore, glass substrates to be used for the FPDs are also currently becoming thinner (formed as a glass film).
Further, there is a growing use of an organic light-emitting diode as a plane light source for interior illumination, which emits only monochrome (for example, white) light. When this type of illumination device includes a glass substrate having flexibility, a light-emitting surface of the illumination device is freely deformable. Therefore, from the viewpoint of ensuring sufficient flexibility, it is also promoted to further thin the glass substrate to be used for the illumination device.
A glass substrate that is thinned to have a thickness of 300 μm or less (glass film) is generally obtained by forming a band-like glass film and then cutting the band-like glass film into a predetermined size. The band-like glass film has flexibility to such a degree that the band-like glass film is not broken even when the band-like glass film is rolled around a roll core, and accordingly the band-like glass film can be rolled into a roll shape to serve as a glass roll. When the band-like glass film is rolled into a roll shape as described above, processing of cutting the band-like glass film into a predetermined width and various kinds of film formation processing can be performed in a roll-to-roll process (process of unrolling one glass roll to obtain the band-like glass film and simultaneously rolling the glass film subjected to various kinds of processing into another glass roll). As a result, production efficiency of displays and illumination devices can be enhanced greatly.
The glass roll to be charged into production equipment (roll-to-roll process) for displays and illumination devices is obtained in the following manner. That is, in a band-like glass film, which is formed of molten glass into a thin sheet shape by an overflow downdraw method or a slot downdraw method, unnecessary portions at both widthwise ends thereof (selvage portions) are cut (primary cutting processing), and the resultant band-like glass film is rolled around a core (see Patent Literature 1). Further, if necessary, the glass roll that is formed and rolled by the above-mentioned method is charged into the roll-to-roll process and cut into a desired width (secondary cutting processing).
For the primary cutting processing and the secondary cutting processing, for example, a cutting method involving laser cleaving is employed. In the method involving the laser cleaving, the band-like glass film is successively cleaved by propagating, along a preset cleaving line extending in a conveyance direction of the band-like glass film, an initial crack formed at a leading end portion of the preset cleaving line using a thermal stress generated in the band-like glass film through localized heating using a laser and cooling of a heated region resulting from the localized heating, which are performed along the preset cleaving line.